1. Field of Invention
The present invention is directed to wireless communication systems. In particular, the present invention is directed to wireless communication systems utilizing multiple antenna arrays.
2. Description of Related Art
Presently, multiple antenna arrays are used for transmitting data in wireless communication systems. For example, multiple antennas are used at both the transmitter and at the receiver for transmitting data. These multiple antenna arrays can increase wireless channel capacity linearly by the number of transmit antennas, providing the number of receive antennas is greater or equal to the number of transmit antennas. Unfortunately, the relatively high dimensional nature of multiple antenna array systems has high computational complexity in practical systems.
One example of a wireless communication system for generating a high data rate using a multiple antenna array is disclosed in U.S. Pat. No. 6,097,771, issued Aug. 1, 2000. This system is otherwise known as a Bell Laboratories Layered Space-Time (BLAST) system. The BLAST system utilizes multiple transmit antennas and multiple receive antennas along with signal processing for interference cancellation.
An example of such systems utilizes, for example, four transmitter antennas and four receiver antennas. This system can create four independent sub-channels within a single bandwidth by coding the sub-channels individually as opposed to coding them jointly. Thus, the system increases capacity within a bandwidth by four-fold by exploiting the idea of diversity such that each channel corresponds with transmitting over many Raleigh fading channels. Accordingly, even if one channel is bad, it is combined with the other good channels to reduce the fluctuation of signal strength. This is because each channel then has enough diversity so that each one ends up appearing as a traditional additive white Gaussian noise channel from clear signal processing. However, the creation of the sub-channels requires signal processing to remove interference between the sub-channels.
In operation, the BLAST system performs signal processing to create four parallel sub-channels in a sequential procedure. This sequential method involves removing channel interference by sequentially subtracting the signal of one sub-channel from the other sub-channels. Unfortunately, the system ends up starting with the sub-channel with the lowest diversity. This is a problem because the sub-channel with the lowest diversity order is, in essence, the worst channel. The higher diversity sub-channels are created by subtracting decisions from the worst sub-channel to create more reliable sub-channels. Therefore, this method is backward in the sense that it starts with the least reliable decision to create successive, more reliable sub-channels. Thus, problems with the worst sub-channel can create problems with the more reliable sub-channels. Also, this method is excessively complex and thus requires a significant amount of processing power. Furthermore, this method has the problem in that it creates unequal levels of diversity. For example, in a four sub-channel system, one sub-channel has a first order diversity, another sub-channel has a second order diversity, another sub-channel has a third order diversity, and the last sub-channel has a fourth order diversity. This creates more unreliability because the lower order diversity sub-channels are more unreliable.